(418d) Effect of Side Chain Length in Phosphoric Acid Based Membranes on Nanophase-Segregation and Transport
Due to the high conductivity and stability, perfluorinated sulfonic acid (PFSA) proton exchange membranes such as Nafion are currently the standard for polymer electrolyte membranes (PEMs). While boasting superior conductive properties at standard conditions, Nafion dehydrates at high temperature and low humidity, drastically compromising its performance. While functionalization of PFSA membranes with phosphoric acid has been shown to improve conductivity under low-humidity conditions, these membranes still require optimization as they lack a practical level of conductivity. In this study, we investigate the nanophase-segregation and transport in phosphoric acid membranes in comparison to Nafion membranes. We also analyze the effect of side chain length on these two properties. To do this, we use density functional theory to prepare the monomeric structures for a sulfonic acid based polymer (Nafion) and a phosphoric acid based polymer. We then build 4 polymer chains with a degree of polymerization of 10 and construct a hydrated, charge-balanced simulation cell. The simulation cells are equilibrated using an annealing molecular dynamics (MD) simulation. After the equilibration, a long-run NPT simulation of 30 ns is performed to collect data for statistical analysis. We then assess nanophase-segregation through various gauges such as pair correlation functions and the scattering factor. Finally, we attempt to estimate how the chemical structures of monomer affect nanophase-segregation and transport. It is found that, as in sulfonic acid membranes, larger water clusters are formed in phosphoric acid membranes with longer side chains. However, unlike in sulfonic acid membranes, the phosphoric acid groups associate closely with one another via hydronium bridging, which suppresses proton hopping. Therefore, it is inferred that the proton transport is decreased with increasing the phosphoric acid side chain length. Based on these results, we recommend opting for shorter side chain lengths when designing phosphoric acid membranes.